1. Field of the Invention
The present invention relates to an optical scanning device functioning as an optical writing unit, an image forming apparatus including the optical scanning device such as a copier, a printer, a facsimile machine, and a plotter, and a liquid crystal element used in the optical scanning device.
2. Description of the Related Art
[Multi-Beam Scanning Device]
In recent years and continuing, image forming apparatuses such as LBPs (laser beam printer) and copiers are forming images of higher quality at higher speed, and color copying is becoming widespread. Accordingly, users are requiring higher quality.
To address the requirements for higher speed, it is effective to employ multi-beams. However, when multi-beams are employed, the pitch (scanning line intervals) between plural beams needs to be adjusted. The pitch between plural beams can be adjusted with a method of rotating a multi-beam light source unit around an optical axis or a method of employing an optical element used for adjusting the pitch (see, for example, Patent Document 1).
To address the requirements for higher quality, beam spots need to have small diameters, and several methods have been proposed (see, for example, Patent Documents 2 through 4). However, in the process of making the diameters of the beam spots small, it is necessary to take into consideration the temperature variations, based on various heat sources such as a fixing unit and a polygon scanner included in an image forming apparatus or an optical scanning device, and temperature variations in the environment.
However, in the conventional method of rotating a multi-beam light source unit around an optical axis, the reliability of electrical components becomes a problem, because the light source unit itself is rotated. The conventional method of employing an optical element used for adjusting the pitch requires a highly precise optical element made of glass, which leads to higher costs.
Even if the beam spot position is initially adjusted in a highly precise manner, when the temperature starts to vary, beam spot positions are displaced with passage of time.
Accordingly, a unit-for adjusting the pitch, i.e., a “liquid crystal element” driven by electric signals, has been proposed. A beam pitch detecting unit provided separately detects the beam pitch, and the liquid crystal element is driven based on the detection results. Thus, when the beam pitch changes with passage of time, the liquid crystal element can correct the change. The liquid crystal element is a beam pitch adjusting unit that can be driven at low voltage, does not generate heat, noise, or vibration, and is compact and light-weight.
[Tandem-Type Color Image Forming Apparatus]
In recent years and continuing, optical elements in scanning optical systems are often made of plastic. Although plastic is advantageous in terms of mass production, final plastic products are often deformed from ideal shapes. The deformation is caused by uneven temperature distributions of the mold when being molded, and/or the plastic products are not uniformly cooled after being removed from the mold.
Scanning optical systems often include optical elements that are longer in a main scanning direction than in a sub scanning direction. Therefore, the optical elements sometimes bend in the sub scanning direction. This may cause positional displacement of scanning lines with respect to the sub scanning direction, such as tilted scanning lines and/or bent scanning lines, depending on how the optical elements are held. Further, errors in mounting an optical element in a housing may also cause considerable positional displacement of scanning lines with respect to the sub scanning direction on the scanning surface.
Further, in an image forming apparatus including plural scanning units, temperature deviations between housings that hold and fix each of the scanning units may cause different amounts of positional displacement of scanning lines with respect to the sub scanning direction, such as bent scanning lines.
There is proposed a method of performing scanning by making plural light beams enter a single optical deflector and arranging optical elements so as to overlap each other in the sub scanning direction (a method of holding all scanning units inside a single optical housing). This method also causes different amounts of positional displacement of scanning lines with respect to the sub scanning direction between photoconductors, such as tilted scanning lines and bent scanning lines. The displacement occurs due to deformation and mounting errors of the scanning optical system, and temperature distributions within a single housing.
In a tandem-type full-color copier, four photoconductive drums are arranged in a row along a conveyance surface of a transfer belt, corresponding to cyan (C), magenta (M), yellow (Y), and black (K). Beam scanning devices scan the photoconductive drums with corresponding beams, thereby forming electrostatic latent images on the circumferential surfaces of the photoconductive drums. The latent images are made visible with toner of corresponding colors. The visible toner images are sequentially transferred onto a recording sheet conveyed by the transfer belt so as to form a full-color image. Thus, if scanning line positions are displaced in the sub scanning direction by different amounts between the colors, image quality is degraded and color deviations occur.    Patent Document 1: Japanese Laid-Open Patent Application No. H9-131920    Patent Document 2: Japanese Laid-Open Patent Application No. H3-116112    Patent Document 3: Japanese Laid-Open Patent Application No. H5-19190    Patent Document 4: Japanese Laid-Open Patent Application No. 2001-166237    Patent Document 5: Japanese Laid-Open Patent Application No. 2004-109700    Patent Document 6: Japanese Laid-Open Patent Application No. H9-131920    Patent Document 7: Japanese Laid-Open Patent Application No. 2004-109699    Patent Document 8: Japanese Laid-Open Patent Application No. 2003-337293
A liquid crystal element has a cell structure in which a liquid crystal layer of several μm to several tens μm in size is sealed by two glass substrates. Therefore, as the temperature rises in the environment, the liquid crystal layer having a relatively high expansion coefficient expands due to heat, and the middle part of the liquid crystal element expands. As a result, a lens effect (positive power) is generated.
Accordingly, beam waist positions are displaced, which disadvantageously leads to larger beam spot diameters.